@agaric the guy who popularised their use was austin osman spare, in the early 1900s! he wrote a lot about them in his books, which are all public domain iirc. unbelievably floral victorian writing tho. i think if you google his name + sigils there's some easier to read guides too!

other ppl do sigils differently to him, but his stuff was the first(ish) and works well for me